In view of escalating fuel demands and diminishing fuel supplies in recent years, it has become increasingly important to conserve the fuel available by reducing consumption or by improving the efficiency of fuel consuming machines. Air conditioning or other cooling systems are devices in which large amounts of fuel can be conserved by relatively small improvements in efficiency or cutbacks in comsumption of each system.
There are many different types of refrigeration systems and some are more energy efficient than others. Therefore, the less efficient systems are gradually being abandoned or replaced at great expense to the system owner. For example, because of energy conservation concerns, absorption refrigeration systems which are typically used for large commercial or industrial air conditioning or other cooling purposes have not been well received in recent years due to the low overall operating efficiencies of such systems. Generally, such absorption units operate at optimum efficiency when the system experiences full load demand and the efficiency drastically decreases as the unit is subjected to partial load conditions. Thus, by modifying absorption units to increase the efficiency at partial load situations, energy producing fuels may be saved and absorption unit owners may avoid the tremendous expense involved in abandoning or replacing the unit.
Most of the refrigeration systems of the absorption type in use today utilize water as a refrigerant and a salt solution such as lithium bromide as the absorbent. These absorption units operate on two basic principles: (1) that water will boil and flash cool at low temperatures if subjected to high vacuum conditions, and, (2) that certain substances absorb water vapor.
The five main components of an absorption refrigeration system generally are the evaporator, the absorber, the solution heat exchanger, the generator, and the condenser. The absorption unit operates on a two pressure cycle; high vacuum condition in the evaporator-absorber section, and a different pressure in the generator-condenser section. A chilled fluid, usually water, is pumped within a closed loop conduit to the evaporator to be cooled indirectly by spray water acting as a refrigerant. In the high vacuum, the refrigerant flashes, thereby cooling the chilled fluid. The refrigerant vaporized in the evaporator is absorbed by a strong absorbent solution, normally lithium bromide, which is pumped through the solution heat exchanger to the generator to reconstitute the weak solution retained within the generator. Using steam or some other heat source, the absorbed refrigerant is boiled off the weak solution and is condensed to a liquid in the condenser section. The refrigerant vapor condensate is returned to the evaporator, while the weak solution, now strong again, flows from the generator to the absorber as an overflow or via the solution heat exchanger arriving as a spray.
Condenser fluid, again usually water, circulates in a closed loop conduit from a cooling tower, through a condenser fluid pump, through the absorber and condenser, and returns to the cooling tower. The circulating condenser fluid removes the heat dissipated in the absorber and condenser.
The chilled fluid, also in a closed loop conduit, circulates from a chilled fluid pump, through the evaporator, to fan units for cooling the air, and returns to the chilled fluid pump.
Capacity controls of one type or another are used to control the absorption unit for partial load conditions. Ordinarily, such control is arranged in one of three ways: (1) throttling the amount of heat supplied to the generator to vary solution concentration, (2) throttling the solution flow from the solution heat exchanger to the generator, thereby changing the solution concentration in the absorber, and (3) throttling the condenser fluid flow to maintain a solution concentration needed to produce refrigeration capacity at the design chilled fluid temperature.
Throttling of the heat source, though not the most economical arrangement, is the most accepted method of control of absorption units. The other two capacity controls also have disadvantages. The cost of the special three-way diverting valve usually used to throttle solution flow is expensive, and the high fluid temperatures resulting at light loads from throttling the condenser fluid accelerates the deposit build-up within the condenser conduit.
Various capacity controls using combinations of the above-mentioned control arrangements and modifications thereof are known. Some control systems for absorption units provide higher operation efficiency, but usually at high initial or operating expense, while other control systems sacrifice efficiency for reduced installation costs.
One system, which controls the temperature in the evaporator, the absorbent solution concentration, and heat input to the generator, utilizes a steam valve responsive to a controller receiving signals from temperature probes in the chilled water entering and leaving the evaporator and in the condenser water leaving the condenser. This system also has a condenser water override valve responsive to the temperature of the condenser water entering the absorber. Although this system does eliminate cycling of the absorption unit by regulating the steam valve, the system is disadvantaged because several expensive electrical controllers and temperature probes are required and the system is not readily conducive to environmental use.
Another control system uses proportional controllers to produce both hot and cold water from an absorption system with a direct-fired high pressure generator. With this system, a pair of controllers which respond to temperatures of the chilled water entering and leaving the evaporator control a fuel control valve, a refrigerant valve and a dilute solution valve. The system is not particularly designed to experience variant load demands, but rather, is designed to use a waste heat source to advantage. The system, also has a high heat rejection to the cooling tower, thereby requiring a larger, more expensive cooling tower.
Still another control system has dual capacity control means to achieve better economy at partial load operation by automatically shutting down one of two absorption refrigeration machines. This system nearly doubles the initial installation and equipment costs involved because two machines are used, yet, no internal change to the general absorption machinery is utilized. Thus, both machines run as usual with no adjustment in operation responsive to partial load demand until the demand is sufficiently low that one machine is shut down. The efficiency of each individual machine is not improved.
In another type of control system, the absorption refrigeration unit has a steam valve responsive to the temperature of the chilled water exiting the evaporator, a condenser water by-pass valve for diverting condenser water from the condenser to the cooling tower, and a three-way valve responsive to the temperature of the condenser water entering the absorber for diverting condenser water from the cooling tower. This system also has multiple electrical contactors for pump start-stop operation which cause high peak demand charges and may cause increased pump maintenance. The control features of this system are expensive and do not significantly improve the efficiency of the absorption unit at variant load demands nor do they improve the reduction of scaling or heat transfer characteristics.
The absorber variant control system of the present invention eliminates many of the problems presented by the above control systems for absorption units. It is an object of this invention to provide more efficient operation of an absorption unit at variant load demands.
Another object of the present invention is to provide effective steam, hot water, or alternate fuel source flow control in proportion to machine load demand.
Still another object of the present invention is to reduce the cooling tower make-up water usage due to reduction of heat load at the condenser, thereby reducing chemical water treatment operating costs.
A further object of the present invention is to provide optimum flow velocities through the evaporator, absorber, and condenser dependent upon cooling load demands and thereby decrease corrosion rates on heat exchanger surfaces, i.e., the absorber, condenser, evaporator and generator, due to a decrease in average operating flow velocities.
Also, an object of the present invention is to provide a proportionate signal to the flow control valves by sensing both the chilled water supply and return temperatures and having a differential temperature controller determine the percent of cooling load demand.
Still another object of the present invention is to reduce both the temperature and concentration of the lithium bromide solution in the generator during low and variant loads by more efficient steam, hot water, or other heat source control.
An additional object of the present invention is to reduce cycling of the machine when the load changes or varies by measuring and comparing the chilled water temperature as it enters and leaves the evaporator and controlling the machine accordingly.
Another object of the present invention is to provide a chilled water flow control valve having a minimum aperture during closing whereby a partial flow is allowed for continuous monitoring for temperature or load changes.
Other objects and advantages of the invention will become apparent upon reading the following summary of the invention, detailed description and appended claims, and upon reference to the accompanying drawings.